IEEE (Institute of Electrical and Electronics Engineers) 802.11 (hereinafter “802.11”) is a family of specifications for wireless local area networks (WLANs), and was developed to maximize interoperability between differing brands of wired local area networks (LANs) as well as to introduce a variety of performance improvements and benefits.
The 802.11 standard defines over-the-air protocols that are necessary to support networking in a local area. A primary service of the 802.11 standard is to deliver MSDUs (MAC Service Data Units) between peer LLCs (Logical Link Controls). Typically, a radio NIC (network interface card) and access point (to be discussed) provide the functions of the 802.11 standard.
The 802.11 standard provides MAC (Media Access Control) and PHY (Physical Layer) functionality for wireless connectivity of fixed, portable, and moving stations moving within a local area. A station is a wireless node that contains functionality of the 802.11 protocol (i.e., MAC layer, PHY layer, and interface to a wireless medium). In its most basic form, stations communicate directly with each other on a peer-to-peer level sharing a given cell coverage area.
The 802.11 topology comprises components that interact to provide a wireless LAN that enables station mobility that is transparent to higher protocol layers such as the LLC, and may support topologies including the IBSS (Independent Basic Service Set) network and the ESS (Extended Service Set) network.
Whereas an IBSS is commonly referred to as an ad hoc network of BSSs, and is often formed on a temporary basis, an ESS comprises a series of BSSs (typically each comprising an AP) that are connected by means of a Distribution System (hereinafter “DS”), which can be almost any type of network, such as an Ethernet LAN, or the Internet, for example. Mobile nodes can then roam between APs, and seamless campus-wide coverage is possible.
Both networks utilize a basic building block referred to as the BSS (Basic Service Set) that provides a coverage area whereby stations of the BSS remain fully connected. A station is free to move within a BSS, but it cannot communicate directly with other stations if it leaves the BSS. Typically, a BSS comprises an access point (hereinafter “AP”). The main function of an AP is to form a bridge between wireless and wired LANs. When an AP is present, stations do not communicate on a peer-to-peer basis. All communications between stations or between a station and a wired network client go through the AP.
The roaming provisions of 802.11 allow for clients to roam among multiple APs that can be operating on the same or separate channel, and provide for APs to transmit beacon signals to their clients. A beacon signal may include a number of parameters, such as a time stamp for client synchronization, a traffic indication map, and an indication of supported data rates, for example. Roaming clients can use the beacon information for a number of purposes. For example, if a client determines that an existing connection is weak, that the APs service has been downgraded, or that an AP is not available when needed, then the client can attempt to associate itself with a new AP.